Pickers are used in agricultural harvesting, in order to separate the crop heads of plants from the other parts of the plants. Such pickers generally have two stalk rolls, between which the plant stalk is pulled in. Occasionally, only one stalk roll is used, which cooperates with a fixed wall. The stalk roll or rolls pulls or pull the plant through a picking gap whose dimensions are narrower than that of the crop head. Because of this, the crop head, for example, an ear of corn, is separated from the rest of the plant and can be processed separately. The stalk rolls are often provided with drivers, mounted on a tubular base element, that extend over their length.
The cylindrical stalk rolls may be provided with forward mounted conical tips provided with helical vanes to pull in the plant, as disclosed in U.S. Pat. No. 4,377,062. In addition, conical stalk rolls have been disclosed, for example, in U.S. Pat. No. 2,870,593, U.S. Pat. No. 3,832,836, DE 1 757 213 A, DE 2 313 519 A, DE 197 30 912 A and EP 0 943 229 A. With these conical stalk rolls their envelopes have a smaller diameter at their front end, in the direction of travel, than at their rear end. Because of this, a situation is achieved in which the plants are pulled in at a relatively slower speed in their lower region, which cooperates with the front sections of the stalk roll that have a smaller diameter. The upper, thinner parts of the plant are pulled in with a relatively higher speed by the rear head region of the stalk rolls that have a larger diameter. Because of this, the cob, which generally grows on the lower part of the plant, is supposed to be pulled in relatively slowly during stripping in the picking gap, and is therefore handled gently, whereas the upper, thin part of the stalk is pulled in rapidly.
In the last two documents mentioned, it is proposed that the axes of the cooperating stalk rolls converge, so that the gap remaining between the stalk rolls can remain a constant width over its length. A picker with conical stalk rolls is also shown in U.S. Pat. No. 1,641,436, whose axes converge so that the width of the gap between the stalk rolls is constant over its length. However, the stalk rolls are thicker at the front, lower end than at the upper, rear head end.
It is considered a shortcoming in pickers with conical stalk rolls that, with parallel stalk rolls, the spacing between the cooperating stalk rolls diminishes progressively to the rear head, leading to crushing of the plant stalks in the rear head region that increases the required drive power, and that the conicity is limited for mechanical-spatial reasons, while in stalk rolls with converging axes, expensive and elaborate angle gear head mechanisms are required. Pickers with cylindrical stalk rolls, unlike those with conical stalk rolls, do not have the advantage of pulling in of the plant that accelerates over the length of the stalk rolls.